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Plant posture can play a key role in the health of aquatic vegetation, by setting drag, controlling light availability, and mediating the exchange of nutrients and oxygen. We study the flow-induced reconfiguration of buoyant, flexible aquatic vegetation through a combination of laboratory flume experiments and theoretical modeling. The laboratory experiments measure drag and posture for model blades that span the natural range for seagrass stiffness and buoyancy. The theoretical model calculates plant posture based on a force balance that includes posture-dependent drag and the restoring forces due to vegetation stiffness and buoyancy. When the hydrodynamic forcing is small compared to the restoring forces, the model blades remain upright and the quadratic law, Fₓ ∝ U², predicts the drag well (Fₓ is drag, U is velocity). When the hydrodynamic forcing exceeds the restoring forces, the blades are pushed over by the flow, and the quadratic drag law no longer applies. The model successfully predicts when this transition occurs. The model also predicts that when the dominant restoring mechanism is blade stiffness, reconfiguration leads to the scaling Fₓ ∝ U 4/3. When the dominant restoring mechanism is blade buoyancy, reconfiguration can lead to a sub-linear increase in drag with velocity, i.e., Fₓ ∝ Ua with a < 1. Laboratory measurements confirm both these predictions. The model also predicts drag and posture successfully for natural systems ranging from seagrasses to marine macroalgae of more complex morphology.

This experimental study describes the turbulent wake behind a two-dimensional porous obstruction, consisting of a circular array of cylinders. The cylinders extend from the channel bed through the water surface, mimicking a patch of emergent vegetation. Three patch diameters ($D$) and seven solid volume fractions ($\\Phi $) are tested. Because flow can pass through the patch, directly downstream there is a region of steady, non-zero, streamwise velocity, ${U}_{1} $, called the steady wake. For the patch diameters and solid volume fractions considered here, ${U}_{1} $ is a function of $\\Phi $ only. The length of the steady wake (${L}_{1} $) increases as $\\Phi $ decreases and can be predicted from the growth of a plane shear layer. The formation of the von-Kármán vortex street is delayed until the end of the steady wake. There are two regions of elevated transverse velocity fluctuation (${v}_{\\mathit{rms}} $): directly behind the patch, associated with the wake turbulence of individual cylinders; and at the distance ${L}_{1} $ from the patch, associated with the formation of large-scale wake oscillation. Velocity along the centreline of the wake starts to increase only after the patch-scale vortex street is formed, and it approaches the free-stream velocity over a distance ${L}_{2} $. The dimensionless length of the entire wake, $({L}_{1} + {L}_{2} )/ D$, increases with patch porosity.

Using a constant channel velocity, U $U$, flume experiments investigated how canopy density (ah $ah$, with canopy frontal area per unit volume a $a$, and canopy height h $h$) and submergence ratio (H/h $H/h$, with H $H$ the flow depth) impacted near‐bed velocity, turbulence, and bedload transport within a submerged canopy of rigid model vegetation. For H/h $H/h$ < 2, the near‐bed turbulent kinetic energy (TKE) was predominantly stem‐generated. As ah $ah$ increased, both the near‐bed TKE and bedload transport rate (qs ${q}_{\\mathrm{s}}$) increased. For H/h $H/h$ > 2, the near‐bed TKE was insensitive to ah $ah$ and H/h $H/h$, because of a trade‐off between decreasing stem‐generated turbulence and increasing canopy‐shear‐generated turbulence, as ah $ah$ and H/h $H/h$ increased. However, the near‐bed velocity declined with increasing ah $ah$ and H/h $H/h$, such that, even with a constant TKE, qs ${q}_{\\mathrm{s}}$ also declined. These trends highlight that both TKE and velocity were important in controlling bedload transport. Models to predict velocity, TKE, and bedload transport were developed and validated with measurements. The models were then used to explore conditions more relevant to the field, specifically with constant energy slope (S $S$) and flexible vegetation. For a constant energy slope, U $U$ increased as ah $ah$ decreased and as H/h $H/h$ increased, which in turn influenced the in‐canopy velocity and TKE. The highest qs ${q}_{\\mathrm{s}}$ occurred with the greatest H/h $H/h$ and smallest ah $ah$, corresponding to the highest U $U$ and greatest contribution of canopy‐shear‐generated turbulence, reflecting the importance of canopy‐shear‐generated turbulence in submerged canopies. The lowest qs ${q}_{\\mathrm{s}}$ occurred with smallest H/h $H/h$ and highest ah $ah$, corresponding to the smallest U $U$ and least contribution of canopy‐shear‐generated turbulence. Plain Language Summary By reducing current, aquatic plants provide many ecosystem services, including nutrient and carbon retention, mitigation of beach and riverbank erosion, and creation of habitat for aquatic organisms. In this study, measurements and modeling were used to define the range of conditions for which submerged vegetation can reduce sediment erosion, relative to unvegetated beds, and specifically reduce the rate at which sediment is carried along the bed (known as bedload transport). In the lab, the channel velocity was held constant, and two regimes were identified. When vegetation extended through more than half of the water depth (water depth/canopy height <2), bedload transport was enhanced compared to unvegetated conditions with the same depth and channel velocity. In contrast, when vegetation extended through less than half of the water depth (water depth/canopy height >2), bedload transport was reduced compared to unvegetated conditions. The lab experiments were used to develop a model to predict bedload transport under field conditions and flexible plants. The model demonstrated that submerged vegetation can diminish erosion, offering a useful guide for river and coastal restoration. Key Points For constant channel velocity, submerged canopies can enhance or reduce bedload transport, depending on their degree of submergence With increasing submergence, the source of near‐bed turbulence shifts from stem wake to the canopy shear layer at the canopy top Bedload transport was best described by a hybrid combination of mean and turbulent velocities

Marsh vegetation, a definitive component of delta ecosystems, has a strong effect on sediment retention and land-building, controlling both how much sediment can be delivered to and how much is retained by the marsh. An understanding of how vegetation influences these processes would improve the restoration and management of marshes. We use a random displacement model to simulate sediment transport, deposition, and resuspension within a marsh. As vegetation density increases, velocity declines, which reduces sediment supply to the marsh, but also reduces resuspension, which enhances sediment retention within the marsh. The competing trends of supply and retention produce a nonlinear relationship between sedimentation and vegetation density, such that an intermediate density yields the maximum sedimentation. Two patterns of sedimentation spatial distribution emerge in the simulation, and the exponential distribution only occurs when resuspension is absent. With resuspension, sediment is delivered farther into the marsh and in a uniform distribution. The model was validated with field observations of sedimentation response to seasonal variation in vegetation density observed in a marsh within the Mississippi River Delta. Wetland vegetation is typically considered only in terms of enhancing sediment accretion and positively impacting land-building. Here, the authors show that the degree of enhancement has a strong dependence on vegetation density through the influence on sediment supply and retention.

Wood is an integral part of a river ecosystem and the number of restoration projects using log placements is increasing. Physical model tests were used to explore how the wood position and submergence level (discharge) affect wake structure, and hence the resulting habitat. We observed a von-Kármán vortex street (VS) for emergent logs placed at the channel center, while no VS formed for submerged logs, because the flow entering the wake from above the log (sweeping flow) inhibited VS formation. As a result, emergent logs placed at the channel center resulted in ten times higher turbulent kinetic energy compared to submerged logs. In addition, both spatial variation in time-mean velocity and turbulence level increased with increasing log length and decreasing submergence level. Submerged logs and logs placed at the channel side created a greater velocity deficit and a longer recirculation zone, both of which can increase the residence time in the wake and deposition of organic matter and nutrients. The results demonstrate that variation in log size and degree of submergence can be used as a tool to vary habitat suitability for different fish preferences. To maximize habitat diversity in rivers, we suggest a diverse large wood placement.

This experimental study describes the mean and turbulent flow structure in the wake of a circular array of cylinders, which is a model for a patch of emergent vegetation. The patch diameter, D, and patch density, a (frontal area per volume), are varied. The flow structure is linked to a nondimensional flow blockage parameter, CDaD, which is the ratio of the patch diameter and a drag length scale (CDa)−1. CD is the cylinder drag coefficient. The velocity exiting the patch, Ue, is reduced relative to the upstream velocity, U∞, and Ue/U∞ decreases as flow blockage (CDaD) increases. A predictive model is developed for Ue/U∞. The wake behind the patch contains two peaks in turbulence intensity. The first peak occurs directly behind the patch and is related to turbulence production within the patch at the scale of individual cylinders. The second peak in turbulence intensity occurs at distance Lwdownstream from the patch and is related to the wake‐scale vortices of the von Karman vortex street. The presence of the flowUe in the wake delays the formation of the von Karman vortex street until distance L1 (

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